I've posted this here before somewhere... at this point, I've posted everything here somewhere and probably shouda' kept half of it to myself.
It ain't near as cold or dry here in Virginia as it is out where you're at, but it starts naturally approaching 30% in my house about this time of year, and even the humidifier cant keep up. So here's a really easy, cheap workaround for bass necks that go berserk in the cold/dry season, or even when the air conditioner messes with things. I've got one, a Gibson, with a mahogany set-neck, that turns into a ski at anything below 45% RH. Acoustic guitars... they really hate getting dry, and not just the neck. The underlying principle is the same... the wood will draw the moisture in naturally if it's available.
Plastic snack boxes. Travel soap boxes are more expensive, but they work well too. (I've got something like 30 guitar cases here with geetars to keep happy in 'em, so cheaper is better in bulk...) Put holes in the top, and fill them with dish sponges, I mean the kind that nobody uses to actually wash dishes with, but they do a nice job of holding water. Put one in the case with your bassesssess. Or put your bassesssses in the case with one. Rotate as necessary.